1. Field of the Invention
The present invention relates to integrated circuit device elements and, in particular, to a dielectric-based anti-fuse cell and cell array, with a polysilicon contact plug and methods for their manufacture.
2. Description of the Related Art
Anti-fuse cells (also known as fusible links) are used as programmable interconnections in logic integrated circuit (IC) devices, such as Field Programmable Gate Arrays (FPGA), and memory IC devices.
A conventional oxide-nitride-oxide (ONO) dielectric-based anti-fuse cell is illustrated in FIG. 1. ONO dielectric-based anti-fuse cell 10 includes an N+ diffusion region 12 disposed on a semiconductor substrate 14. ONO dielectric layer 16 and doped polysilicon layer 18 are disposed above the N+ diffusion region 12. The doped polysilicon layer 18 and the N+ diffusion region 12 are separated by both the ONO dielectric layer 16 and an insulating layer 20, except in one region where the separation is accomplished solely by the ONO dielectric layer 16. Insulating layer 20 is also disposed on the upper surface of semiconductor substrate 14. As illustrated in FIG. 1, a portion of the N+ diffusion region 12 lies under a gap in the insulating layer 20, with the doped polysilicon layer 18 and its underlying ONO dielectric layer 16 filling the gap.
The ONO dielectric-based anti-fuse cell 10 of FIG. 1 is programmed to a conductive state by applying a voltage across the ONO dielectric layer 16 (i.e. the voltage is applied between the N+ diffusion region 12 and doped polysilicon layer 18 at the xe2x80x9cgapxe2x80x9d in the insulating layer 20) sufficient to rupture the ONO dielectric layer at the gap. Additional details of ONO-based anti-fuses, such as the anti-fuse cell illustrated in FIG. 1, are described in E. Hamdy et al., Dielectric Based Antifuse for Logic and Memory ICs, Technical Digest of the International Electron Devices Meeting, 786-787 (1988), which is hereby fully incorporated by reference.
A conventional dual-polysilicon and nitride-oxide (NO) dielectric-based anti-fuse cell 30 for use in FPGA applications is illustrated in FIG. 2. Anti-fuse cell 30 includes a doped polysilicon (poly 1) layer 32 that is separated from semiconductor substrate 34 by first insulating layer 36 (typically silicon dioxide). A second insulating layer 38 (typically silicon dioxide), with an opening therethrough, is disposed on the upper surface of poly 1 layer 32. A portion of the upper surface of the poly 1 layer 32 is exposed through the opening in the second insulating layer 38. A layer of NO dielectric 40 is disposed on the surface of second insulating layer 38, including on the sidewalls of the opening, and on the portion of the poly 1 layer that is exposed through the opening. A second doped polysilicon (poly 2) layer 42 overlies the layer of NO-dielectric 40 and further fills the opening in the second insulating layer 38.
The dual-polysilicon NO dielectric-based anti-fuse cell of FIG. 2 is programmed to a conductive state by applying a voltage across the layer of NO dielectric 40 (i.e. the voltage is applied between the poly 1 layer 32 and the poly 2 layer 42 at the opening in the second insulating layer 38) sufficient to rupture the NO dielectric at the opening. The use of an NO dielectric layer, rather than an ONO dielectric layer, reduces the voltage required to rupture the dielectric. See David K. Y. Liu et al., Scaled Dielectric Antifuse Structure for Field Programmable Gate Array Applications, IEEE Electron Device Letters, Vol. 12, No. 4, 151-153 (1991), which is hereby fully incorporated by reference, for a further description of dual-polysilicon NO dielectric-based anti-fuse elements.
Prior to being programmed, a dielectric-based anti-fuse cell is in a high resistance state due to the presence of a dielectric layer (either ONO or NO) separating the conductive layers (either a doped diffusion region in a semiconductor substrate or a doped polysilicon layer). After the anti-fuse cell is programmed (i.e. the dielectric is ruptured) by the application of a voltage (xe2x80x9canti-fuse programming voltagexe2x80x9d), the anti-fuse cell assumes a relatively low resistance state and therefore becomes more conductive. In this xe2x80x9cprogrammedxe2x80x9d state, the resistance of the anti-fuse cell is dependent on the area and electrical properties of the contact between the conductive layers. The series resistance of a cell array is proportional to the length of the polysilicon lines connecting individual anti-fuse cells of the array.
Reducing (i.e. xe2x80x9cscalingxe2x80x9d) the anti-fuse programming voltage of dielectric-based anti-fuses often requires thinning the dielectric layer. However, thinning the dielectric layer increases the anti-fuse capacitance, which causes an undesirable decrease in integrated circuit device speed (i.e. an increase in the time delay). The capacitance of the anti-fuse in the non-programmed state can be reduced by minimizing the contact area of the anti-fuse. However, when the contact area of a conventional anti-fuse is reduced, the resistance of the anti-fuse is increased, resulting in a reduction in device speed. In addition, there is a reduction in device speed due to the preexistence of undesirable anti-fuse cell array capacitance associated with the polysilicon lines employed to connect the individual cells of the array (e.g. capacitance between polysilicon lines and the substrate).
Still needed in the art is a dielectric-based anti-fuse cell, and a process for its manufacture, that allows for the area of the anti-fuse dielectric to be scaled along with the dielectric thickness, while still providing an anti-fuse cell with a low capacitance and a low resistance in the programmed state. It is desirable that the dielectric-based anti-fuse have a relatively small contact area, thereby decreasing anti-fuse capacitance while providing for an increased cell layout density with low series resistance. Also needed is a anti-fuse cell array with a low resistance in the programmed state, a low capacitance, a small cell area and high cell density. The process of manufacture for the cell and cell array should be simple and fully compatible with conventional Complimentary-Metal-Oxide-Semiconductor (CMOS) processing.
The present invention provides a dielectric-based anti-fuse cell, having a doped polysilicon contact plug, with a low resistance in the programmed state, a low capacitance, and a small cell area.
The anti-fuse cell includes a first insulating layer, typically SiO2, on the surface of a semiconductor substrate such as a silicon wafer. A first doped polysilicon (poly 1) layer is disposed on the upper surface of the first insulating layer and a second insulating layer covers the poly 1 layer.
The anti-fuse cell also includes a doped polysilicon contact plug that extends from the upper surface of the second insulating layer, through the second insulating layer and into the poly 1 layer. A dielectric layer, such as an ONO dielectric composite layer or an NO dielectric composite layer, covers the upper surface of the doped polysilicon contact plug. A second doped polysilicon (poly 2) layer is disposed on the dielectric layer. The poly 1 and poly 2 layers can optionally include a layer of metal silicide on their upper surfaces.
This anti-fuse structure provides a low capacitance anti-fuse cell since the contact area between the dielectric layer and the doped polysilicon contact plug can be manufactured at the minimum dimension which the manufacturing process is capable of producing. The structure also provides an anti-fuse cell with a low resistance in the programmed state since the doped polysilicon contact plug is electrically connected to the poly 1 layer along the relatively large sidewall area of the doped polysilicon contact plug. The area of the anti-fuse cell, according to the present invention, can be reduced to a minimum since its doped polysilicon contact plug can be produced to the smallest size possible for a given process technology. The small anti-fuse cell area provides for a compact array layout with short polysilicon lines connecting the individual cells, decreasing the series resistance and capacitance of the cell array.
Also provided is a process for manufacturing a dielectric-based anti-fuse cell with a doped polysilicon contact plug that is simple (and therefore inexpensive) and compatible with conventional CMOS processing.
The process includes first providing a semiconductor substrate and forming a first insulating layer on its surface. Next a first doped polysilicon (poly 1) layer is formed on the surface of the first insulating layer followed by the formation of a second insulating layer over the poly 1 layer.
A contact opening is then created that extends from the upper surface of, and through, the second insulating layer and into the poly 1 layer. A doped polysilicon plug layer (i.e. the precursor to the doped polysilicon contact plug) is then formed on the upper surface of the second isolating layer and in the contact opening. The doped polysilicon plug layer is subsequently removed everywhere except in the contact opening, thereby forming a doped polysilicon contact plug in the contact opening.
Next, a dielectric layer, such as an ONO or NO layer, is formed on the upper surface of the doped polysilicon contact plug and the second insulating layer, followed by the formation of a second doped polysilicon (poly 2) layer on the upper surface of the dielectric layer.
Each of these process steps can be accomplished utilizing conventional CMOS processing techniques. Furthermore, a relatively small number of additional steps are needed compared to manufacturing processes of conventional dielectric-based anti-fuse cells.